ImGui canvas with 4 draggable control points (p0/p3 locked at (0,0)/(1,1)
by default for use as easing curves). Pure evaluation via De Casteljau
(bezier_point) plus sampling-based y(x) lookup (bezier_eval).
Render uses fn_tokens for visual coherence: bg, border, primary curve,
text_dim diagonal reference, text_muted handle tangents.
p1.x and p2.x clamped to [0,1] to keep the curve usable as easing; Y
values are free to allow deliberate overshoot.
egutierrez
a8e153b73f